The present invention relates generally to hydrostatic power steering systems, and more particularly, to such hydrostatic power steering systems in which multiple fluid controllers are used to control the flow of fluid from a source of pressurized fluid to a fluid pressure actuated device, such as a steering cylinder.
A typical prior art hydrostatic power steering system includes a fluid reservoir, a fluid pressure source (such as a power steering pump), a single fluid controller, and a fluid pressure actuated device (such as a steering cylinder). The fluid controller used in such a steering system is typically a full fluid-linked steering controller. While this type of steering system is used in many hydraulic applications, it is limited to those hydraulic applications which require only one fluid controller location from which to control the fluid pressure actuated device.
Some applications, however, including but not limited to marine applications, require that the fluid pressure actuated device be controllable from multiple locations on a given application. Although a given application could include more than two locations from which to control the fluid pressure actuated device, the present invention will be described, for ease of description, in regard to applications requiring only two locations from which to control the fluid pressure actuated device, without intending to limit the present invention in any way. In order to accommodate this two-location requirement, a multiple-input (i.e., two or more steering wheels) hydrostatic power steering system is typically used. The typical prior art multiple-input hydrostatic power steering system used in such applications includes a fluid reservoir, a fluid pressure source (such as a power steering pump), two fluid controllers, and a fluid pressure actuated device.
Typically, in applications using a multiple-input hydrostatic power steering system, the fluid controller used in the first location is a “power beyond” fluid controller. In addition to an inlet port, a return port, and a pair of control fluid ports, a typical power beyond fluid controller also includes an additional fluid port through which inlet fluid is diverted when the fluid controller is in the neutral position. This diverted inlet fluid then flows through fluid connectors and fluid hoses to the inlet port of a second fluid controller, disposed in the second location on the application. The second fluid controller used in the second location is usually of the open-center type. While such a steering system allows for multiple location controllability of a fluid pressure actuated device, the cost of such a system becomes prohibitive in some applications. Two areas of the typical multiple-input hydrostatic power steering system which account for significant cost premiums are the additional fluid connectors and hoses needed to divert inlet fluid from the first fluid controller to the second fluid controller and the use of two substantially different fluid controllers in the first and second locations.
In order to reduce the cost of the previously described steering system, some applications have replaced the power beyond fluid controller with an open center fluid controller which diverts inlet fluid through the internal cavity (also referred to as “case”) and return port of the fluid controller to the fluid inlet of the fluid controller in the second location. While the described steering system does allow for multiple location controllability of a fluid pressure actuated device, the life of the shaft seal and the thrust bearings in the fluid controller in the first location is reduced due to the force exerted on those components from the pressurized inlet fluid which is diverted through the internal cavity of the fluid controller. Therefore, the use of an open center fluid controller in the first location is unacceptable in many applications as a result of such potential decreased life.